The present invention relates to an elastomeric suspension spring, usable for bearing the load of a motor vehicle, which spring exhibits both reduced creep and improved endurance in torsion. The invention also relates to a suspension joint comprising this spring and a process for reducing the creep of this spring when it is incorporated into said joint on a vehicle.
Motor vehicle suspension joints generally use blocks which each comprise an elastomeric spring for, as compared to rolling or plain bearings which were used in the past, improved integration of load-bearing functions and shock or vibration absorption and filtering. A suspension device comprising elastomeric suspension springs has the advantage of comprising a reduced total number of components, in particular due to the fact that it does not comprise a separate filtering block for linking an axle or a suspension arm to the vehicle body.
Use of elastomeric springs also provides the corresponding suspension device with the required degrees of freedom, while ensuring satisfactory filtering, in particular from an acoustic point of view. Moreover, it is possible to impart to these elastomeric springs quite sophisticated guiding functions by controlling their deformation under in service stresses, to achieve for example self-steering axles, i.e. axles which turn the plane of the wheels slightly solely by transferring forces, or to incorporate a horizontal degree of freedom.
International Patent Application WO-A-97/47486 discloses a suspension device, the axle of which is provided with elastomeric suspension and anti-roll springs, which are stressed torsionally. Suspension springs, which are intended to bear the load of the vehicle, control wheel clearance relative to a bearing point formed by the vehicle body. Anti-roll springs in turn control wheel clearance (or the clearance of the arm bearing the wheel) relative to a bearing point consisting of the opposing wheel (or the opposing arm, respectively).
U.S. Pat. No. 4,383,074 discloses a natural rubber-based elastomeric spring composition which is designed to impart to a spring satisfactory dynamic functioning endurance.
This composition comprises, in a quantity of less than or equal to 40 phr (phr: parts by weight per hundred parts of elastomer), a reinforcing filler which consists of a blend of two carbon blacks, one being a very fine 200-grade black (reinforcing grade “ISAF” black) and the other being a “coarse” 700-grade black (non-reinforcing grade “SRF” black). In the examples given in this document, the reinforcing filler comprises said 200-grade black in a mass fraction greater than 45% and said 700-grade black in a mass fraction of less than 55%. This filler thus comprises a relatively high proportion of reinforcing grade carbon black, to obtain the above-mentioned endurance.
This composition is furthermore obtained by means of a sulfur cross-linking system, with a quantity of sulfur equal to 2.75 phr and a mass ratio of sulfur to cross-linking accelerator equal to 2.75.
U.S. Pat. No. 5,904,220 discloses, in its example embodiments, a natural rubber-based elastomeric spring composition which also comprises a reinforcing filler consisting of reinforcing grade carbon black. This reinforcing filler is present in said composition in a quantity of 55 phr.
This composition is likewise obtained by means of a sulfur cross-linking system, with a quantity of sulfur equal to 0.5 phr and a ratio of sulfur to cross-linking accelerators of approximately 0.14.
None of these patent specifications relates to the technical problem of creep in elastomeric spring compositions, which is a major disadvantage for currently known suspension joints and which manifests itself after they are fitted to the vehicle axles, in particular after a few days of dynamic operation of these joints in torsion (said creep then being known as dynamic creep). This creep results in changes to the height of the vehicle body over time.
The Applicant has surprisingly discovered a cross-linked rubber composition that may be used to produce an elastomeric suspension spring which, when it is installed between two coaxial armatures to bear the load of a motor vehicle, advantageously exhibits minimized dynamic creep and satisfactory torsional endurance, the cross-linked rubber composition being based on natural rubber in a quantity equal to or greater than 60 phr (phr: parts by weight per hundred parts of diene elastomer(s)) and based on a sulfur cross-linking system comprising 0.7 to 1.2 phr of sulfur and comprising at least one cross-linking accelerator such that the mass ratio of sulfur/cross-linking accelerator(s) varies from 0.15 to 2.70, and based on a filler comprising, in a mass fraction greater than 55%:                a carbon black, whose grade varies from 600 to 900, said composition comprising in this case a total quantity of filler of from 10 to 60 phr, or        an inert white filler, said composition comprising in this case a total quantity of filler of from 10 to 30 phr.        
Of course, the expression “composition based on” is to be understood to mean a composition comprising the mix and/or the product of reaction in situ of the different constituents used, some of these compounds being able to and/or being intended to react together, at least in part, during the different phases of manufacture of the composition.
The natural rubber may be used in a blend with one or more other “essentially unsaturated” diene elastomers in the rubber composition according to the invention, which may be present in a quantity less than or equal to 40 phr.
Diene elastomer is understood to mean, in known manner, an elastomer (a homopolymer or a copolymer) resulting at least in part from diene monomers (monomers bearing two double carbon-carbon bonds, whether conjugated or not).
In general, “essentially unsaturated” diene elastomer is understood here to mean a diene elastomer resulting at least in part from conjugated diene monomers, this elastomer having a content of members or units of diene origin (conjugated dienes) which is greater than 15% (mol %).
These definitions being given, the following are understood in particular to be meant by diene elastomer(s) capable of being used in a blend with natural rubber in the compositions according to the invention:
any homopolymer obtained by polymerization of a conjugated diene monomer having 4 to 12 carbon atoms;
any copolymer obtained by copolymerization of one or more dienes conjugated together or with one or more vinyl aromatic compounds having 8 to 20 carbon atoms;
The diene elastomer of the composition according to the invention is selected for example from the group of diene elastomers consisting of polybutadienes (BR), polyisoprenes (IR) or butadiene-styrene copolymers (SBR).
Preferably the composition according to the invention comprises a blend:
of natural rubber in a quantity of from 70 phr to 100 phr, and
of synthetic polyisoprene in a quantity of from 30 phr to 0 phr.
Even more preferably, the composition according to the invention comprises solely natural rubber (that is to say in a quantity of 100 phr).
As far as the filler comprising the composition according to the invention is concerned, said filler preferably comprises, in a mass fraction greater than 70%, said carbon black whose grade varies from 600 to 900 or said inert white filler.
It is possible to use as the filler of the composition according to the invention a blend of said carbon black whose grade varies from 600 to 900 and said inert white filler.
It is also possible to use as filler of said composition either said carbon black whose grade varies from 600 to 900 or said inert white filler (the above-mentioned mass fraction being in this case equal to 100%).
According to a preferred example of embodiment of the invention, said filler comprises, in the above-mentioned mass fraction (greater than 55% or preferably than 70%), said 600- to 900-grade carbon black (that is to say a “coarse” black of a grade known as being non-reinforcing). As carbon blacks of this type which may be used in the composition according to the invention, mention may be made, for example, of 700-grade carbon blacks, such as those known as “N772” or “N765”.
Preferably, the filler of said composition comprises said 600- to 900-grade carbon black in a quantity of from 40 to 60 phr. It will be noted that such a quantity of carbon black is particularly well adapted to imparting satisfactory thermoelasticity to the corresponding joint, i.e. which is representative of a variation in attitude minimized as a function of the temperature gradients to which it is subjected.
According to one variant embodiment of the invention, the filler of the composition according to the invention comprises, in the above-mentioned mass fraction (greater than 55% or more preferably greater than 70%), an inert white filler. As inert white filler (that is to say an inorganic filler which has little or no reinforcing action, sometimes also known as inert clear filler), which is usable as all or part of said filler, a silica with little reinforcing action is used, for example, whose BET and CTAB specific surface area values are close to 60 m2/g, such as silica sold under the name “ULTRASIL 360”. It is also possible to use, as non-reinforcing white filler, clays, such as kaolin or bentonite, or chalks, or alternatively titanium oxides.
The filler of said composition may comprise said inert white filler in a quantity of from 10 to 30 phr. It will be noted that such a quantity of inert white filler is particularly well adapted to imparting satisfactory cold stiffening to the composition according to the invention (cold stiffening is known to mean an increase in static stiffness in the event of diminishing temperature).
Of course, the filler of the composition according to the invention may comprise a plurality of carbon blacks ranging in grade from 600 to 900 and/or a plurality of inert white fillers.
According to a preferred example of embodiment of the invention, the sulfur cross-linking system used is a system in which the mass ratio of sulfur to cross-linking accelerator(s) varies from 0.15 to 0.50.
It will be noted that this preferred range for the value of the sulfur/accelerator ratio allows minimization of the secondary creep of the composition according to the invention (secondary creep is understood to mean creep of a chemical nature resulting from changes to the cross-linking network).
Advantageously, the cross-linking activator used is zinc 2-ethylhexanoate. It will be noted that this specific activator also allows minimization of the dynamic creep of the elastomeric spring made of the composition according to the invention.
The elastomeric spring according to the invention is for example obtained:
by mixing by thermomechanical working of the various constituents of the rubber composition according to the invention, which is performed in one or more stages in an internal mixer, followed by cross-linking of the cross-linkable composition thus obtained in an injection mould using techniques known to the person skilled in the art.
According to another feature of the invention, said cross-linked composition has a dynamic shear modulus G* at 100% deformation, measured to standard ASTM D 5992-96 at a temperature of 23° C. and at a frequency of 10 Hz to standard ASTM D 1349-99, which is within a range from 0.5 MPa to 5.5 MPa.
The present invention also provides a suspension joint for a motor vehicle, said joint being intended to bear the load of said vehicle and consisting of two substantially cylindrical and concentric armatures which are connected together by an elastomeric suspension spring.
According to the invention, this joint is such that said suspension spring is such as defined above.
The present invention also provides a process for reducing the creep of an elastomeric suspension spring usable for bearing the load of a motor vehicle, when said spring is fitted between two coaxial armatures with a view to bearing the load of a motor vehicle, said spring consisting of a cross-linked rubber composition.
According to the invention, this process consisting substantially in mixing by thermomechanical working:                one or more diene elastomers, which comprise natural rubber in a quantity equal to or greater than 60 phr;        a filler which comprises, in a mass fraction greater than 55%;        a carbon black, whose grade varies from 600 to 900, said filler being in this case present in the composition in a quantity of from 10 to 60 phr; or        an inert white filler, said filler being in this case present in said composition in a quantity of from 10 to 30 phr, and        a sulfur cross-linking system comprising at least one cross-linking accelerator, said cross-linking system comprising from 0.7 to 1.2 phr of sulfur and being such that the mass ratio of sulfur to cross-linking accelerator(s) varies from 0.15 to 2.70.        
The aforementioned features of the present invention, as well as others, will be better understood on reading the following description of several examples of embodiment of the invention, which are given by way of illustration and not of limitation, said description being made in conjunction with the attached drawings, in which: